Mass spectrometers are devices allowing the chemical structure of the constituent molecules of a sample or analyte to be characterized. Mass spectrometry is thus a micro-analysis technique that typically only requires a few picomoles of the sample in order to extract from it characteristic information regarding its molecular weight or its molecular structure. There exist various types of mass spectrometers, amongst which may be noted mainly time-of-flight mass spectrometers, quadrupole mass spectrometers and magnetic mass spectrometers. Reference may for example be made to a general publication such as that by John Roboz, Introduction to mass spectrometry, Instrumentation and techniques, published by Interscience publishers, 1968, or else by J. Throck Watson, Introduction to Mass Spectrometry, published by Lippincott-Raven, 1997, notably introducing the various types of mass spectrometers and their principles of operation. Magnetic mass spectrometers may be further separated into spectrometers with single focusing and spectrometers with double focusing. As far as the theoretical aspects relating to the optical properties of mass spectrometers are concerned, reference may be made to the work by H. Wollnik, Optics of charged particles, published by Academic Press, 1987.
In the following, the qualification “optical” is to be considered as it is accepted in its wider sense, here applied to ion optics.
One particular type of mass spectrometry is commonly denoted by the name SIMS, which is an acronym for the expression “Secondary Ion Mass Spectrometry”. One of the problems specific to this technique of analysis is that the ions accelerated in the mass spectrometer exhibit a large energy dispersion. Regarding the chromatic properties of mass spectrometers, and notably the devices involving ions exhibiting a large energy dispersion, reference may usefully be made to the work by A. Benninghoven et al., Secondary Ion Mass Spectrometry, published by John Wiley, 1987. This publication notably deals with SIMS techniques.
The present invention falls notably into the field of mass spectrometers of the SIMS type. In spectrometers of this type, it is known that the principle for extraction of the secondary ions leads to a large dispersion in energy of the emitted ions. It is furthermore known that an electrostatic section may advantageously be introduced into mass spectrometers of the SIMS type between the sample and the magnetic section, this electrostatic section being designed to render the mass spectrometer achromatic for at least one mass. It is commonly possible to vary the magnetic field produced by the magnetic section. This is easily achieved by varying the electrical excitation, for example in the case where the magnetic field is produced by an electromagnet. In this case, the condition for achromatism is not associated with a given mass, but with a particular trajectory. If this trajectory is considered as the main axis of the spectrometer, the spectrometer is said to be achromatic on the axis, and not to be achromatic away from the axis or “off-axis”.
Still more particularly, the present invention may relate both to a spectrometer referred to as “single-collection”, in other words capable of measuring a mass along the axis, and to a mass spectrometer referred to as “multi-collection”, in other words capable of measuring several masses simultaneously. It is for example possible to simultaneously measure several masses by disposing a plurality of collectors in the focal plane of the mass spectrometer. The blur observed at the focal point of a given mass different from the on-axis mass, when the energy distribution of the ions is relatively broad, is called off-axis chromatic aberration. Employing the notation of H. Wollnik, presented in the aforementioned publication, this blur may be characterized by an aberration coefficient x/em defined by the equation:Δx1=(x/em)×(ΔE/E)×(M1−M0)/M0,
where M0 is the mass on the main axis for which there is good chromatic focusing, ΔE the energy dispersion of the beam, and Δx 1 the blur formed at the place where the trajectories of the mass M1 are focused at the opening.
It is desired to reduce the blur Δx 1 in order to improve the off-axis mass resolution. In order to reduce this blur, the aim is to cancel the coefficient x/em. In multi-collection mass spectrometers, it is therefore necessary, in order to guarantee a good resolution in mass for various masses, to be able to eliminate or significantly reduce the off-axis chromatic aberrations.
Various types of mass spectrometers exist known from the prior art, which are constructed so as to be achromatic on the axis. Amongst these types of spectrometers, the Nier-Johnson spectrometer may be mentioned.
The Mattauch-Herzog spectrometer, also known from the prior art, is notably characterized by the fact that the exit face of the magnet is aligned with the entry point. This particular configuration enables a certain number of noteworthy properties, and notably allows achromatism to be obtained for various masses. However, it is sometimes very advantageous for a mass spectrometer to have a large dispersion in mass, and in this case, the Mattauch-Herzog spectrometer is not suitable.
It is furthermore known that it is preferable, with the aim of increasing the resolution in mass, to eliminate or reduce the aberrations of the second order. It is recalled here that the use in mass spectrometers of elements that are not axisymmetric about the main axis, such as the electrostatic and magnetic sections, leads to aberrations of the second order. These aberrations cannot, by definition, be corrected by a focusing process. Four types of aberrations of the second order are produced in mass spectrometers comprising a magnetic section and an electrostatic section; these aberrations of the second order are denoted according to the usage in the field of optics or of ion optics: a first aberration denoted x/aa proportional to the square of the opening angle in the radial plane, a second aberration x/bb proportional to the square of the opening angle in the transverse plane, a third aberration x/ae proportional to the opening angle in the radial plane and to the relative difference in energy, and a fourth aberration x/ee proportional to the square of the relative difference in energy.
It is known that the respective geometrical parameters of the electrostatic section and of the magnetic section and of other ion optics devices may be calculated in such a manner that the 4 second order aberration coefficients cancel each other out. For example, reference may be made to the work by H. Matsuda, Double focusing mass spectrometer of second order, International Journal of Mass Spectrometry and Ion Processes, 14 (1974). In this publication, a spectrometer with double focusing designed with a set of very precisely determined physical and geometrical parameters is notably proposed. This type of solution has several drawbacks: there is no possibility of adjusting the correction for the aberrations and, if the calculations are not completely exact, the aberrations are not really canceled.
Furthermore, this type of spectrometer cannot be differently adjusted according to the type of performance specifications that it is desired to favor: for example, a very good resolution in mass only on the axis or else a reasonably good resolution in mass for all the masses detected by the multi-collection. Lastly, this type of spectrometer is not stigmatic, in other words it is impossible to dispose at the exit of the spectrometer an ion microscope function which enables an image of the sample, filtered in mass, to be displayed.
It is also known that hexapoles can correct aberrations of the second order. Reference may, for example, be made to the aforementioned work by Wollnik. A hexapole is a set of six poles disposed about the main axis, and alternately biased at an electrical potential of +V or −V.
Spectrometers known from the prior art are equipped with correcting electrostatic hexapoles: a mass spectrometer with single focusing as described in the European patent application EP 0124440 or a mass spectrometer with double focusing such as described in the U.S. Pat. No. 4,638,160. The advantage of introducing electrostatic hexapoles in order to reduce the aberrations is that it is then possible to adjust the aberration correction as finely as possible by adjusting the excitation voltage of the hexapole while at the same time observing a signal characteristic of the sharpness of the spot, such as for example the signal resulting from the scanning of the beam over the edge of the exit slit of the spectrometer, which exit slit is disposed upstream of a counting mechanism or the projection onto an ion-photon conversion device, such as a micro-channel wafer, of the image of the ion beam in the plane of the exit slit.
In a mass spectrometer with stigmatic double focusing, it is known that as long as a difference in magnification between the image in the radial plane and the transverse plane is not created, it is not possible to simultaneously cancel the aforementioned first and second aberrations of the second order x/aa and x/bb; but it is also known that if this difference in magnification is created with the appropriate means such as described in the European patent application EP0473488, it is then possible to simultaneously cancel these two aberrations.
Reference may be made to the article by E. de Chambost et al., Achieving High Transmission with the Cameca IMS1270, Secondary Mass Spectrometry, SIMSX, published by John Wiley, 1995, where it is notably stated that with a hexapole situated between the entry slit and the electrostatic section, a hexapole situated upstream of the magnetic section and a hexapole situated downstream of the magnetic section, the aforementioned first three aberrations of the second order x/aa, x/bb and x/ae may be canceled. This configuration allows the transmission for a resolution in mass of the order of 10,000 to be considerably improved. However, the fourth aberration of the second order x/ee is not canceled, which represents a serious drawback when resolutions in mass greater than 20,000 are required.